Reverse power protection circuit and relay

ABSTRACT

A relay for high frequency instruments and a reverse power protection circuit for high frequency instruments are described herein. The relay is inexpensive to construct, permits good impedance matching up to approximately 5 GHz, and can be easily mounted on a coplanar waveguide transmission line. The circuit can open the relay in response to a reverse power condition in as little as 8 microseconds and provides both good grounding and functions reliably up to 4 GHz.

This is a divisional of copending application Ser. No. 08/609,151 filedon Feb. 29, 1996, now U.S. Pat. No. 5,684,441.

BACKGROUND OF THE INVENTION

This invention relates to circuits and devices for protecting radiofrequency ("RF") instruments from accidental injection of high power RFsignals. In particular, the present invention is a reverse powerprotection ("RPP") limiter/detector circuit and relay. The relay mountseasily on a printed circuit board ("PCB") and operates reliably up toapproximately 5 GHz. The RPP limiter/detector circuit functions well upto 4 GHz.

RF instruments such as signal generators, spectrum analyzers, networkanalyzers, and measuring receivers may be exposed to high power RFsignals of 50 W or more if such signals are accidentally applied to theinstruments' external signal port. The sensitive internal circuitry ofthese instruments can be damaged when exposed to such high powersignals. To protect the internal circuitry, RPP limiter/detectorcircuits and relays are used.

Typically, limiting diodes coupled to the external signal port as shuntswith a predetermined bias voltage, permitting nominal signal levels in aforward direction but limiting any reverse signal input to thepredetermined voltage level. The size of the diodes is limited by theneed to match impedances along the signal path, thereby minimizingsignal reflection and resulting signal degradation over the operatingfrequency range. Small diodes have only a small capacitance andtherefore affect the overall impedance of the circuit less. Given theirsmall size, these diodes can only protect the internal circuitry fromhigh power signals for a short time. After this short time, the diodesfail, exposing the instrument to the high power RF signal.

To increase the protection of the RF instrument's internal circuitry, arelay is placed in the signal path between the RF output connector andthe limiting diodes. Normally, the relay is closed, allowing signals toflow in either direction. In response to the injection of a reversepower signal above a predetermined threshold, the relay is triggeredopen by an RPP limiter/detector circuit. The open relay saves thelimiting diodes and internal circuitry from damage.

The limiting diodes provide an interim time period for protecting theinternal circuitry while the reverse power surge is detected by the RPPlimiter/detector circuit and the relay switched open. The diodes, therelay and its coupling capacitors, and RPP limiter/detector circuit musttogether closely match the impedance of the RF instrument to avoidsignal reflection and related signal degradation during normaloperation.

Known RPP limiter/detector circuits and relays use microstriptransmission lines for both the diodes and transmission line structures.The series inductance created by the lengthy connection between thesurface mounted diodes and the ground plane makes the upper frequencylimit of these printed circuit microstrip designs approximately 3.5 Ghz.To fabricate RPP limiter/detector circuits that will work at frequenciesabove 4 Ghz, the path length to ground must be reduced. This haspreviously been accomplished by using thin circuit material and amicrocircuit design with the diode chip bonded onto the microstrip. Suchmicrocircuits are typically more expensive than PCBs and are only usedwhen the desired performance can not be achieved any other way. Even thebest known RPP limiter/detector circuits and relays using thick filmmicrocircuits and wire bonds perform relatively poorly above 2 GHz,providing reduced power protection at these higher frequencies, anddoing so at relatively high cost.

SUMMARY OF THE INVENTION

A first preferred embodiment of the present invention comprises a lowcost coaxial relay and a very fast RPP limiter/detector circuit. Therelay comprises a glass reed switch, elastomeric conductive tube, outermetal ground shield, magnetic coil and bobbin assembly, and outerhousing. The ground shield provides very low impedance connections tothe ground planes used in the present invention and the entire relay canbe assembled for a much lower cost than known coaxial relays. The RPPlimiter/detector circuit that the relay is coupled to can sense areverse power signal very quickly and can open the coaxial relay within8-10 microseconds("μsecs") of the application of the reverse powersignal. The use of a high voltage zener diode and high voltagetransistors in the RPP limiter/detector circuit enable this fastresponse time.

The present invention will be discussed with reference to the figureslisted and described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b are, respectively, a side and a cross-sectional view ofthe metal tube used in the present invention's relay;

FIG. 2 is a side cross-sectional view of the bobbin used in the presentinvention's relay;

FIG. 3 is a side cross-sectional view of the present invention'scompleted relay;

FIG. 4 cross-sectional view of a coplanar waveguide transmission line;

FIG. 5 is a top plan view of the coplanar waveguide and PCB assemblyused in the present invention; and

FIGS. 6A and 6B are a schematic drawings of the RPP circuit used in thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The Relay

Coaxial relay 10 (see FIG. 3) described herein is a low cost relay witha match of 20 dB return loss and a 1.22:1 VSWR up to and beyond 4 GHz.Relay 10 maintains the characteristics of a 50 Ω transmission line up toand beyond 4 GHz when surface mounted on a PCB. When coupled to RPPlimiter/detector circuit 100 (see FIG. 6), relay 10 can be opened in aslittle as 6 microseconds ("μs")

To obtain these performance characteristics, the outer shield connectionof relay 10 must be carefully matched to the PCB. A low inductanceconnection between outer shield 12 (see FIG. 1a) of relay 10 is enabledby extending 2 shield contacts 14 from each end of relay shield 12.These are soldered directly to the PCB.

At least one known relay wrapped a foil shield around the glass reedswitch used to open and close the relay. The foil left air gaps aroundthe solid glass ends of the reed switch, resulting in impedancevariations among different relays. This was partially solved by the useof an elastomeric conductive tube in place of the foil shield, as taughtin U.S. Pat. No. 5,258,731 ("'731"). Previously known PCB relays alsohad a long lead at each end for the coaxial ground connection. Theincreased ground impedance caused by the long leads resulted inperformance degradation above 2 Ghz.

Relay 10 (see FIG. 3) is formed by creating a stepped impedance, lowpass filter structure. Reed switch 16 is formed so that reed switch wire18 is fully encased in glass at both ends of reed switch 16. In themiddle of reed switch 16, the contacts are surrounded by air. Sections21, where reed switch wire 18 is encapsulated by glass, form a lowimpedance transmission line. Section 23, where reed switch wire 18 issurrounded by air, forms a high impedance transmission line. Thus reedswitch 16 has a low impedance section 21, a high impedance section 23,and a second low impedance section 21. This series of sections forms alow pass filter.

The impedance value of low impedance sections 21 must be consistent fromone relay to the next. Therefore, as in the '731 patent, conductiveelastomer tube 25 is used to cover glass reed switch 16. Tube 25conforms to switch 16 and minimizes any potential air gaps, creating aconsistent outer shield diameter, which results in a consistentimpedance value.

To assemble the relay, metal tube 12 (see FIGS. 1a and 1b) is formedfrom brass and tin plated with two ground contacts 14 at both ends oftube 12. Slot 35 is cut through the length of tube 12 and prevents tube12 from becoming a shorted turn, preventing current flow in tube 12 andreducing the opening time of the completed relay. In the absence ofcurrent flow, the magnetic field which holds the contact wires togethercannot be maintained, and the relay opens. Elastomer tube 25 (see FIG.3), substantially identical to that described in the '731 patent, isplaced within tube 12. Glass reed switch 16 is then pressed into thecombined assembly of metal tube 12 and elastomer tube 25. A magneticcoil and bobbin assembly 41 (see FIGS. 2 and 3) is then placed inside aplastic housing 51. The combination of reed switch 16, elastomer tube25, and metal tube 12 is passed through a hole 53 in plastic housing 51and through a hole 43 in bobbin 41 until it is centered in thehousing/bobbin assembly. The completed assembly is then encapsulatedusing a known epoxy compound to prevent the various components fromshifting in position. To complete the assembly of the relay, thininsulating washers 55, made from kapton, are placed over each end ofelastomeric tube 25 to prevent tube 25 from shorting out to the centerconductor of the PCB coplanar conductor (see FIG. 4). This conductormust be extended as close to the relay as possible, to minimize theinductance of the connection between the relay and the conductor.

Completed relay 10 costs roughly $7.00 in volume production, whichcontrasts with the $130.00 cost of the '731 relay, provides a very good50 Ω impedance match up to at least 5 GHz with a 20 dB return loss, andcan be surface mounted on a PCB. The finished relay's package height isalso critical to its proper functioning. By keeping the height to withina predetermined height above and below the PCB, the creation ofundesired modes within the relay's pass band is eliminated. The twoground contacts 14 allow for good ground contact and a good impedancematch between the PCB and relay 10. Equally important, they facilitatebalancing the electromagnetic field at either side of the coplanarwaveguide.

The Reverse Power Protection Limiter/Detector Circuit

FIG. 4 is a side view of a coplanar waveguide transmission line 70constructed with a center conductor 71 surrounded by 2 ground planes 73.The coplanar waveguide transmission line shown in FIG. 4 is known andsimilar to that used in the present invention.

FIG. 5 illustrates the coplanar waveguide transmission line and portionsof the RPP limiter/detector circuit used in the present invention. Inknown PCB fabrication processes, the conductive material is etched andthen plated up. To obtain the desired performance in the presentinvention, much tighter process controls are used in the fabrication ofthe PCB than have been typically used before. This permits much tightertolerances. In the preferred embodiments, the width of the conductor andground planes can be controlled to within +/-1/2 mill and the tracethickness and the spaces between the ground planes and traces can becontrolled to within +/-3/10 mill. Conductor 81 has RF connectors 87 and89 at its respective ends. The distance between ground planes 83 andconductor 81 varies along its length. Between connector 87 and relay 10,the distance narrows from 19 mills at connector 87 to 12 mills at relay10. Between grounding patches 82 and conductor 81, the distance is 11.5mills.

Diodes 85 are coupled between central conductor 81 and ground planes 83in a reverse bias configuration so that they do not conduct under normalconditions and affect the signal output. The capacitance of diodes 85must be matched to that of the transmission line to maintain properimpedance matching. Making the diode capacitance part of a low passfilter structure is one way of facilitating this matching process. Also,the length of the connection between the conductor and ground includingthe diode path length must also be kept as short as possible to minimizeparasitic impedances. Given these constraints, diodes 85 must be ACcoupled to ground planes 83. The AC connection must provide a goodground connection up to at least 4 GHz, and preferably up to 5 GHz.

Grounding patch 82 on both sides of conductor 81 are isolated from therest of ground planes 83. Patches 82 must be kept as small as possibleto avoid the propagation of undesirable modes. A plurality of capacitors86, capacitors 86 having different values, are coupled between groundingpatches 82 and ground planes 83. Capacitors 86 vary in value from 51picofarad("pF") to 680 pF. Each grounding patch also has a slightlydifferent total capacitance value coupling it to ground planes 83, tominimize any resonances associated with the capacitors and thepropagation of undesired modes. The smaller valued capacitors providehigh frequency grounding and the larger valued capacitors provide lowfrequency grounding. The exact values of the capacitor can be varied asnecessary for different frequency ranges. Grounding patches 82 are alsonot directly across from one another. Known methods are used tocalculate the necessary lateral distance. Also, as the grounding patchesthemselves add resonance to the RPP circuit, grounding patches 82 arekept as small as possible.

Additional AC grounding between grounding patches 82 and ground planes83 is provided by adding ground layers(not illustrated) in other innerlayers of the PCB. Many vias are added to couple all the additionalground layers and ground planes 83 together. After that coupling occurs,grounding patches 82 comprise a parallel plate capacitor formed withground planes 83 on the second layer of the PCB. This combination ofground planes and vias provides an excellent ground at frequencies up to4 GHz.

Relay 10's ground contacts 14(see FIGS. 1a and 5) are attached directlyto ground planes 83 without the use of vias. In known reverse powerprotection circuits, narrow contact strips couple the relay's groundconnections to the microstrip transmission line. These contact stripsadded inductance and limit the high frequency performance of thecircuit. In the present invention, the direct coupling of the relay tothe ground planes introduces less inductance and improves high frequencyperformance.

To prevent ground imbalances and modes occurring between the groundplanes of the coplanar waveguide transmission line, narrow straps(notillustrated) on the back side of the circuit board couple the groundplanes together.

The connection between SMA coaxial connectors 87 and 89 and the PCB ofthe present invention also have reduced inductance when compared to theknown art as the outer conductor of the coaxial connector is coupleddirectly to the ground planes. As shown in FIGS. 5 and 6, capacitors 91couple the sections of conductor 81 between relay 10 and diodes 85together. They function as DC blocking capacitors and allow a higher DCvoltage to be applied to the input of the RPP limiter/detector circuitwithout causing the circuit to trip. In this embodiment, capacitors 91are mounted on their sides for proper impedance matching.

Referring now to FIG. 6, during normal operation relay 10 is closed andan output signal travels from RF₋₋ IN to RF₋₋ OUT. In a reverse powersituation, when the signal level on the RF₋₋ OUT line reaches 6.2 V,peak diodes 85 begin to conduct, raising the voltage across transientabsorbers ("Transorbs") 92 and 93. Transorbs 92 and 93 and diodes 85limit the incoming reverse power waveform to +/-7.7 V. They can absorbup to 600 W for 1 millisecond. The bias point on transorbs 92 and 93 isset as high as possible. This speeds up the circuit and increases theprotection level by pre-charging the capacitance of transorbs 92 and 93.

As the voltage across transorbs 92 and 93 increases, voltage divider101, comprised of resistors 103, 104, and 105, generates an outputvoltage signal, which is applied to the negative input of comparator110. In this embodiment, only the positive side of the reverse powersignal is used to provide an input to the RPP limiter/detector circuit.In other embodiments, a similar peak detector could be used to detectthe negative side of the reverse power signal exclusively, or both sidesof the reverse power signal could be detected by having both a positiveand negative peak detector. Circuit 120 provides a temperaturecompensated very stable voltage threshold signal to the positive inputof comparator 110. As the output voltage signal from voltage divider 101increases above the threshold voltage supplied by circuit 120,comparator 110 senses the increase and, when it exceeds the thresholdvoltage, generates a low output. The threshold voltage of comparator 110is set above the bias voltage on transorbs 92 and 93. The higher thebias voltage on transorbs 92 and 93, the more they can be pre-charged,which provides better electro-static discharge ("ESD") protection. Theoutput of comparator 110 is applied to a set-reset flip-flop made up ofNAND gates 130 and 140. When the output of the flip-flop goes high, theoutput of NAND gate 141 goes low. This in turn turns transistor 151 on,which turns off transistor 153. Turning off transistor 153 stops currentflow through the coil of relay 10, which then opens. Zener diode 171,which has a very high threshold voltage of 160 V, allows relay 10 toopen very quickly. The more energy that is absorbed in diode 171'selectric field, the less current is available to flow into relay 10'smagnetic coil. As the current is reduced, the magnetic field weakens,allowing the contacts to open faster. The entire process, from the timethe signal level increases to the time that the relay contacts open,requires 8-10 μsecs. During the time it takes for the relay to opentransorbs 92 and 93 limit the amplitude of the reverse power signal.One-shot reset circuit 160 is comprised of comparators 161 and 163. Oncerelay 10 has been opened, a reset signal must be received by the RPPlimiter/detector circuit on reset line 170 to close relay 10 again. Ifthe reverse power signal that caused relay 10 to open is still present,one-shot reset circuit 160 allows the RPP limiter/detector circuit tore-open relay 10, even if the reset signal is continuously asserted.One-shot reset circuit 160 feeds only one narrow pulse to the set-resetflip-flop of the RPP limiter/detector circuit each time the reset signalis asserted.

The present invention has shown lower insertion loss, higher frequencyresponse, and better impedance matching over known microstriptransmission line printed circuit designs. The present invention alsoprotects against higher levels of reverse power than known circuits. Thepresent invention has exhibited better than 20 dB return loss(1.22:1VSWR) at frequencies up to 4 GHz.

What is claimed is:
 1. A reverse power protection circuit comprising:afirst and second reverse biased diodes coupled respectively between asignal line and a first and second AC grounding patch; a first andsecond plurality of capacitors coupled between the AC grounding patchand a first and second ground plane; a first and second transientabsorber coupled respectively between the AC grounding patch and theground plane; a resistive voltage divider network coupled to at leastone of the AC grounding patches and a first comparator, the firstcomparator having a predefined voltage trigger threshold; a flip-flopcircuit for providing a logic signal output when the threshold of thefirst comparator is exceeded; a first transistor coupled through aninverter to the flip-flop circuit, the first transistor acting as aswitch and changing state in response to the logic signal output of theflip-flop circuit; a second transistor coupled to the first transistor,the second transistor acting as a switch and changing state in responseto the first transistor's changing state; and a relay having an inputand output, the relay being opened and closed by the de-energizing andenergizing of a electromagnetic coil in the relay, the coil beingde-energized when the second transistor changes state.
 2. The reversepower protection circuit of claim 1 wherein a one-shot circuit iscoupled to the flip-flop circuit, the one-shot circuit preventing theflip-flop circuit from oscillating after it is first triggered if areverse power signal is being received when a reset signal is applied tothe reverse power protection circuit.
 3. The reverse power protectioncircuit of claim 1 wherein the second transistor further comprises ahigh voltage transistor and a high voltage zener diode coupled inparallel across the high voltage transistor, the combination decreasingthe time required to open the relay.